1. Field of the Invention
The present invention is in the field of hydraulic pumps, and relates particularly to the type of hydraulic pump having a rotating cylinder block and wherein inlet and output porting is accomplished by cooperation between cylinder porting in the rotating cylinder block with fixed porting in the housing, this type of hydraulic pump finding particular utility for fuel injection systems.
2. Description of the Prior Art
According to the current state of the art of hydraulic systems, most hydraulic pumps are of the rotating cylinder block type. In this type of pump, means in the body of the pump such as a swash plate is engageable with the pistons which orbit with the rotating cylinder block for moving the pistons in regular succession through positive pressure or compression strokes and negative pressure or suction strokes, with the pistons in one 180.degree. sector of the body moving in positive pressure or compression strokes and the pistons in the other 180.degree. sector of the body moving in negative pressure or suction strokes. Of the rotating cylinder block pumps, it is estimated that at least about 95 percent employ kidney port type inlet and output valving.
In a kidney port-type valve system, two large arcuate, kidney-shaped ports are stationary in the pump body, carried in an end housing of the pump body. One of the large kidney ports covers one 180.degree. sector for intake, and the other covers the other 180.degree. sector for discharge. These large stationary kidney ports are separated by lands at their ends, so that they are actually each somewhat less than 180.degree. in extent. An annular array of small, regularly spaced cylinder kidney ports is located in a flat annular cylinder port face on the rotating cylinder block, each communicating with a respective pump cylinder. The rotating cylinder port axis and the axis of the large stationary kidney ports are coaxial, and the small cylinder ports commutate abruptly from positive pressure to negative pressure at one land, and from negative pressure to positive pressure at the other land.
The state-of-the-art kidney port-type valve employed in rotating cylinder block hydraulic pumps has a number of serious problems which are inherent in the geometry, and particularly in the coaxial alignment of the rotating cylinder ports and the large stationary kidney ports. One such inherent problem is that the large stationary kidney ports and rotating cylinder ports are required to have a relatively large radial spacing from the axis of rotation relative to the specified flow volume and pressure of the pump, which causes an undesirably high scrubbing velocity of the rotating cylinder block port face against the opposed stationary kidney port face, resulting in an undesirably large amount of wear.
Another inherent problem with the kidney port-type valving is that the long stationary kidney-shaped slots with their abrupt ends and the short intermediate lands make flat lapping of the opposed porting surfaces a problem in production, tending to result in an inaccurate face-to-face seal between the valving surfaces. The relatively large porting radius contributes to this difficulty of flat lapping in production.
The abrupt intersecting of the rotating cylinder block ports with the lands at the ends of the long stationary kidney ports also results in serious problems. There is an abrupt 180.degree. opposing geometry of the end edges of the small rotating ports relative to the end edges of the large stationary ports, which results in very rapid opening and closing functions of the porting. This abrupt 180.degree. opposing geometry of the ports is compounded as a problem by the undesirably large radial disposition of the ports from the axis of rotation, which results in high relative approach and departure speeds of the ports.
One such problem is that this abrupt 180.degree. intersecting of the rotating cylinder port edges with the large kidney port lands has a tendency to cause grooving in one or both of the valving surfaces, with consequent hydraulic fluid leakage from between the valving surface interface.
The positive and negative pumping functions of the rotating cylinder block pump, where the pistons are swash plate actuated, are essentially sine functions, with zero pumping at the lands, and with full positive or pressure pumping proximate the center of one of the large stationary kidney ports and full negative or suction pumping proximate the center of the other large stationary kidney port. The abrupt exposure of essentially zero pressure cylinder ports to full high pressure output and full negative pressure input of the pump inherently introduces undesirable pressure irregularities or "hydraulic ripple" into both the output and the input, and input irregularities will be reflected as further irregularities in the output.
Inaccuracies in the flatness of the opposed valving surfaces of kidney port-type rotating cylinder block pumps resulting from manufacturing difficulties and from wear such as grooving tend to cause leak paths in the axial direction normal to the general planes of the valving surfaces, while the relatively large perimeter of the kidney port valving surfaces required for the specified flow volume and pressure of the pump are cumulative factors which tend to result in a relatively large fluid escape path in the radial direction, or parallel to the general planes of the valving surfaces. A serious consequence of these leakage factors is that they result in a relatively slow build-up of pump output pressure at low pump speeds. Since fuel injector pumps are engine driven, this can be a critical deficiency at engine starting speeds, with engine starting being totally dependent upon pump pressure build-up.